[c#] center MessageBox in parent form

Is there a easy way to center MessageBox in parent form in .net 2.0

I really needed this in C# and found Center MessageBox C#

Here's a nicely formatted version

using System;
using System.Windows.Forms;
using System.Text;
using System.Drawing;
using System.Runtime.InteropServices;   

public class MessageBoxEx
{
    private static IWin32Window _owner;
    private static HookProc _hookProc;
    private static IntPtr _hHook;

    public static DialogResult Show(string text)
    {
        Initialize();
        return MessageBox.Show(text);
    }

    public static DialogResult Show(string text, string caption)
    {
        Initialize();
        return MessageBox.Show(text, caption);
    }

    public static DialogResult Show(string text, string caption, MessageBoxButtons buttons)
    {
        Initialize();
        return MessageBox.Show(text, caption, buttons);
    }

    public static DialogResult Show(string text, string caption, MessageBoxButtons buttons, MessageBoxIcon icon)
    {
        Initialize();
        return MessageBox.Show(text, caption, buttons, icon);
    }

    public static DialogResult Show(string text, string caption, MessageBoxButtons buttons, MessageBoxIcon icon, MessageBoxDefaultButton defButton)
    {
        Initialize();
        return MessageBox.Show(text, caption, buttons, icon, defButton);
    }

    public static DialogResult Show(string text, string caption, MessageBoxButtons buttons, MessageBoxIcon icon, MessageBoxDefaultButton defButton, MessageBoxOptions options)
    {
        Initialize();
        return MessageBox.Show(text, caption, buttons, icon, defButton, options);
    }

    public static DialogResult Show(IWin32Window owner, string text)
    {
        _owner = owner;
        Initialize();
        return MessageBox.Show(owner, text);
    }

    public static DialogResult Show(IWin32Window owner, string text, string caption)
    {
        _owner = owner;
        Initialize();
        return MessageBox.Show(owner, text, caption);
    }

    public static DialogResult Show(IWin32Window owner, string text, string caption, MessageBoxButtons buttons)
    {
        _owner = owner;
        Initialize();
        return MessageBox.Show(owner, text, caption, buttons);
    }

    public static DialogResult Show(IWin32Window owner, string text, string caption, MessageBoxButtons buttons, MessageBoxIcon icon)
    {
        _owner = owner;
        Initialize();
        return MessageBox.Show(owner, text, caption, buttons, icon);
    }

    public static DialogResult Show(IWin32Window owner, string text, string caption, MessageBoxButtons buttons, MessageBoxIcon icon, MessageBoxDefaultButton defButton)
    {
        _owner = owner;
        Initialize();
        return MessageBox.Show(owner, text, caption, buttons, icon, defButton);
    }

    public static DialogResult Show(IWin32Window owner, string text, string caption, MessageBoxButtons buttons, MessageBoxIcon icon, MessageBoxDefaultButton defButton, MessageBoxOptions options)
    {
        _owner = owner;
        Initialize();
        return MessageBox.Show(owner, text, caption, buttons, icon,
                               defButton, options);
    }

    public delegate IntPtr HookProc(int nCode, IntPtr wParam, IntPtr lParam);

    public delegate void TimerProc(IntPtr hWnd, uint uMsg, UIntPtr nIDEvent, uint dwTime);

    public const int WH_CALLWNDPROCRET = 12;

    public enum CbtHookAction : int
    {
        HCBT_MOVESIZE = 0,
        HCBT_MINMAX = 1,
        HCBT_QS = 2,
        HCBT_CREATEWND = 3,
        HCBT_DESTROYWND = 4,
        HCBT_ACTIVATE = 5,
        HCBT_CLICKSKIPPED = 6,
        HCBT_KEYSKIPPED = 7,
        HCBT_SYSCOMMAND = 8,
        HCBT_SETFOCUS = 9
    }

    [DllImport("user32.dll")]
    private static extern bool GetWindowRect(IntPtr hWnd, ref Rectangle lpRect);

    [DllImport("user32.dll")]
    private static extern int MoveWindow(IntPtr hWnd, int X, int Y, int nWidth, int nHeight, bool bRepaint);

    [DllImport("User32.dll")]
    public static extern UIntPtr SetTimer(IntPtr hWnd, UIntPtr nIDEvent, uint uElapse, TimerProc lpTimerFunc);

    [DllImport("User32.dll")]
    public static extern IntPtr SendMessage(IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);

    [DllImport("user32.dll")]
    public static extern IntPtr SetWindowsHookEx(int idHook, HookProc lpfn, IntPtr hInstance, int threadId);

    [DllImport("user32.dll")]
    public static extern int UnhookWindowsHookEx(IntPtr idHook);

    [DllImport("user32.dll")]
    public static extern IntPtr CallNextHookEx(IntPtr idHook, int nCode, IntPtr wParam, IntPtr lParam);

    [DllImport("user32.dll")]
    public static extern int GetWindowTextLength(IntPtr hWnd);

    [DllImport("user32.dll")]
    public static extern int GetWindowText(IntPtr hWnd, StringBuilder text, int maxLength);

    [DllImport("user32.dll")]
    public static extern int EndDialog(IntPtr hDlg, IntPtr nResult);

    [StructLayout(LayoutKind.Sequential)]
    public struct CWPRETSTRUCT
    {
        public IntPtr lResult;
        public IntPtr lParam;
        public IntPtr wParam;
        public uint message;
        public IntPtr hwnd;
    } ;

    static MessageBoxEx()
    {
        _hookProc = new HookProc(MessageBoxHookProc);
        _hHook = IntPtr.Zero;
    }

    private static void Initialize()
    {
        if (_hHook != IntPtr.Zero)
        {
            throw new NotSupportedException("multiple calls are not supported");
        }

        if (_owner != null)
        {
            _hHook = SetWindowsHookEx(WH_CALLWNDPROCRET, _hookProc, IntPtr.Zero, AppDomain.GetCurrentThreadId());
        }
    }

    private static IntPtr MessageBoxHookProc(int nCode, IntPtr wParam, IntPtr lParam)
    {
        if (nCode < 0)
        {
            return CallNextHookEx(_hHook, nCode, wParam, lParam);
        }

        CWPRETSTRUCT msg = (CWPRETSTRUCT)Marshal.PtrToStructure(lParam, typeof(CWPRETSTRUCT));
        IntPtr hook = _hHook;

        if (msg.message == (int)CbtHookAction.HCBT_ACTIVATE)
        {
            try
            {
                CenterWindow(msg.hwnd);
            }
            finally
            {
                UnhookWindowsHookEx(_hHook);
                _hHook = IntPtr.Zero;
            }
        }

        return CallNextHookEx(hook, nCode, wParam, lParam);
    }

    private static void CenterWindow(IntPtr hChildWnd)
    {
        Rectangle recChild = new Rectangle(0, 0, 0, 0);
        bool success = GetWindowRect(hChildWnd, ref recChild);

        int width = recChild.Width - recChild.X;
        int height = recChild.Height - recChild.Y;

        Rectangle recParent = new Rectangle(0, 0, 0, 0);
        success = GetWindowRect(_owner.Handle, ref recParent);

        Point ptCenter = new Point(0, 0);
        ptCenter.X = recParent.X + ((recParent.Width - recParent.X) / 2);
        ptCenter.Y = recParent.Y + ((recParent.Height - recParent.Y) / 2);


        Point ptStart = new Point(0, 0);
        ptStart.X = (ptCenter.X - (width / 2));
        ptStart.Y = (ptCenter.Y - (height / 2));

        ptStart.X = (ptStart.X < 0) ? 0 : ptStart.X;
        ptStart.Y = (ptStart.Y < 0) ? 0 : ptStart.Y;

        int result = MoveWindow(hChildWnd, ptStart.X, ptStart.Y, width,
                                height, false);
    }

}

I have changed a little bit previous answer and compose WPF version of the MessageBoxEx. This code works for me great. Feel free to notify about issues of the code.

Please note: I use GeneralObjects.MainWindowInstance at ctor to initialize class with my main window, but actually I use it for any window due to some kind of cache for last parent window. Therefore you can simple remove out everything from ctor.

public class MessageBoxEx
{
    private static HwndSource source_ = null;
    private static HwndSourceHook hook_ = null;

    static MessageBoxEx()
    {
        try
        {
            // create cached 
            createHwndSource_(GeneralObjects.MainWindowInstance);

            hook_ = new HwndSourceHook(HwndSourceHook);
        }
        finally
        {
            if (null == source_ ||
                null == hook_)
            {
                source_ = null;
                hook_ = null;
            }
        }


    }

    private static void createHwndSource_(Window owner)
    {
        source_ = (HwndSource)PresentationSource.FromVisual(owner);
    }

    public static void Initialize_(Window owner = null)
    {
        try
        {
            if (null != owner)
            {
                if(source_.RootVisual != owner)
                {
                    createHwndSource_(owner);
                }

            }
        }
        finally
        {
            if (null == source_ ||
                null == hook_)
            {
                source_ = null;
                hook_ = null;
            }
        }


        if (null != source_ &&
            null != hook_)
        {
            source_.AddHook(hook_);
        }

    }

    public static MessageBoxResult Show(string messageBoxText)
    {
        Initialize_();
        return System.Windows.MessageBox.Show(messageBoxText);
    }


    public static MessageBoxResult Show(string messageBoxText, string caption)
    {
        Initialize_();
        return System.Windows.MessageBox.Show(messageBoxText, caption);
    }


    public static MessageBoxResult Show(Window owner, string messageBoxText)
    {
        Initialize_(owner);
        return System.Windows.MessageBox.Show(owner, messageBoxText);
    }


    public static MessageBoxResult Show(string messageBoxText, string caption, MessageBoxButton button)
    {
        Initialize_();
        return System.Windows.MessageBox.Show(messageBoxText, caption, button);
    }


    public static MessageBoxResult Show(Window owner, string messageBoxText, string caption)
    {
        Initialize_(owner);
        return System.Windows.MessageBox.Show(owner, messageBoxText, caption);
    }


    public static MessageBoxResult Show(string messageBoxText, string caption, MessageBoxButton button, MessageBoxImage icon)
    {
        Initialize_();
        return System.Windows.MessageBox.Show(messageBoxText, caption, button, icon);
    }


    public static MessageBoxResult Show(Window owner, string messageBoxText, string caption, MessageBoxButton button)
    {
        Initialize_(owner);
        return System.Windows.MessageBox.Show(owner, messageBoxText, caption, button);
    }


    public static MessageBoxResult Show(string messageBoxText, string caption, MessageBoxButton button, MessageBoxImage icon, MessageBoxResult defaultResult)
    {
        Initialize_();
        return System.Windows.MessageBox.Show(messageBoxText, caption, button, icon, defaultResult);
    }


    public static MessageBoxResult Show(Window owner, string messageBoxText, string caption, MessageBoxButton button, MessageBoxImage icon)
    {
        Initialize_(owner);
        return System.Windows.MessageBox.Show(owner, messageBoxText, caption, button, icon);
    }


    public static MessageBoxResult Show(string messageBoxText, string caption, MessageBoxButton button, MessageBoxImage icon, MessageBoxResult defaultResult, System.Windows.MessageBoxOptions options)
    {
        Initialize_();
        return System.Windows.MessageBox.Show(messageBoxText, caption, button, icon, defaultResult, options);
    }


    public static MessageBoxResult Show(Window owner, string messageBoxText, string caption, MessageBoxButton button, MessageBoxImage icon, MessageBoxResult defaultResult)
    {
        Initialize_(owner);
        return System.Windows.MessageBox.Show(owner, messageBoxText, caption, button, icon, defaultResult);
    }


    public static MessageBoxResult Show(Window owner, string messageBoxText, string caption, MessageBoxButton button, MessageBoxImage icon, MessageBoxResult defaultResult, System.Windows.MessageBoxOptions options)
    {
        Initialize_(owner);
        return System.Windows.MessageBox.Show(owner, messageBoxText, caption, button, icon, defaultResult, options);
    }

    private enum WM : int
    {
        WM_ACTIVATE = 0x0006
    }

    private static IntPtr HwndSourceHook(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
    {

        if ((int)WM.WM_ACTIVATE == msg &&
            source_.Handle == hwnd &&
            0 == (int)wParam)
        {

            try
            {
                CenterWindow(lParam);
            }
            finally
            {
                // remove hook at once after moved message box window.
                source_.RemoveHook(hook_);
            }
        }
        return IntPtr.Zero;
    }

    [DllImport("user32.dll")]
    private static extern bool GetWindowRect(IntPtr hWnd, ref Rectangle lpRect);


    [DllImport("user32.dll")]
    private static extern int MoveWindow(IntPtr hWnd, int X, int Y, int nWidth, int nHeight, bool bRepaint);

    private static void CenterWindow(IntPtr hChildWnd)
    {
        System.Drawing.Rectangle recChild = new System.Drawing.Rectangle(0, 0, 0, 0);

        bool success = GetWindowRect(hChildWnd, ref recChild);

        int width = recChild.Width - recChild.X;
        int height = recChild.Height - recChild.Y;

        System.Drawing.Rectangle recParent = new System.Drawing.Rectangle(0, 0, 0, 0);
        success = GetWindowRect(source_.Handle, ref recParent);

        System.Drawing.Point ptCenter = new System.Drawing.Point(0, 0);
        ptCenter.X = recParent.X + ((recParent.Width - recParent.X) / 2);
        ptCenter.Y = recParent.Y + ((recParent.Height - recParent.Y) / 2);


        System.Drawing.Point ptStart = new System.Drawing.Point(0, 0);
        ptStart.X = (ptCenter.X - (width / 2));
        ptStart.Y = (ptCenter.Y - (height / 2));

        // I have commented this code because of I have 2 monitors
        // so If application located at 1st monitor
        // message box can appear at second one.

        /*
        ptStart.X = (ptStart.X < 0) ? 0 : ptStart.X;
        ptStart.Y = (ptStart.Y < 0) ? 0 : ptStart.Y;
        */

        int result = MoveWindow(hChildWnd, ptStart.X, ptStart.Y, width,
                                height, false);

    }


}

But why stop with MessageBox-specific implementation? Use the class below like this:

    private void OnFormClosing(object sender, FormClosingEventArgs e)
    {
        DialogResult dg;
        using (DialogCenteringService centeringService = new DialogCenteringService(this)) // center message box
        {
            dg = MessageBox.Show(this, "Are you sure?", "Confirm exit", MessageBoxButtons.YesNo, MessageBoxIcon.Question);
        }

        if (dg == DialogResult.No)
        {
            e.Cancel = true;
        }
    }

The code that you can use with anything that shows dialog windows, even if they're owned by another thread (our app has multiple UI threads):

(Here is the updated code which takes monitor working areas into account, so that the dialog isn't centered between two monitors or is partly off-screen. With it you'll need enum SetWindowPosFlags, which is below)

public class DialogCenteringService : IDisposable
{
    private readonly IWin32Window owner;
    private readonly HookProc hookProc;
    private readonly IntPtr hHook = IntPtr.Zero;

    public DialogCenteringService(IWin32Window owner)
    {
        if (owner == null) throw new ArgumentNullException("owner");

        this.owner = owner;
        hookProc = DialogHookProc;

        hHook = SetWindowsHookEx(WH_CALLWNDPROCRET, hookProc, IntPtr.Zero, GetCurrentThreadId());
    }

    private IntPtr DialogHookProc(int nCode, IntPtr wParam, IntPtr lParam)
    {
        if (nCode < 0)
        {
            return CallNextHookEx(hHook, nCode, wParam, lParam);
        }

        CWPRETSTRUCT msg = (CWPRETSTRUCT)Marshal.PtrToStructure(lParam, typeof(CWPRETSTRUCT));
        IntPtr hook = hHook;

        if (msg.message == (int)CbtHookAction.HCBT_ACTIVATE)
        {
            try
            {
                CenterWindow(msg.hwnd);
            }
            finally
            {
                UnhookWindowsHookEx(hHook);
            }
        }

        return CallNextHookEx(hook, nCode, wParam, lParam);
    }

    public void Dispose()
    {
        UnhookWindowsHookEx(hHook);
    }

    private void CenterWindow(IntPtr hChildWnd)
    {
        Rectangle recChild = new Rectangle(0, 0, 0, 0);
        bool success = GetWindowRect(hChildWnd, ref recChild);

        if (!success)
        {
            return;
        }

        int width = recChild.Width - recChild.X;
        int height = recChild.Height - recChild.Y;

        Rectangle recParent = new Rectangle(0, 0, 0, 0);
        success = GetWindowRect(owner.Handle, ref recParent);

        if (!success)
        {
            return;
        }

        Point ptCenter = new Point(0, 0);
        ptCenter.X = recParent.X + ((recParent.Width - recParent.X) / 2);
        ptCenter.Y = recParent.Y + ((recParent.Height - recParent.Y) / 2);


        Point ptStart = new Point(0, 0);
        ptStart.X = (ptCenter.X - (width / 2));
        ptStart.Y = (ptCenter.Y - (height / 2));

        //MoveWindow(hChildWnd, ptStart.X, ptStart.Y, width, height, false);
        Task.Factory.StartNew(() => SetWindowPos(hChildWnd, (IntPtr)0, ptStart.X, ptStart.Y, width, height, SetWindowPosFlags.SWP_ASYNCWINDOWPOS | SetWindowPosFlags.SWP_NOSIZE | SetWindowPosFlags.SWP_NOACTIVATE | SetWindowPosFlags.SWP_NOOWNERZORDER | SetWindowPosFlags.SWP_NOZORDER));
    }

    // some p/invoke

    // ReSharper disable InconsistentNaming
    public delegate IntPtr HookProc(int nCode, IntPtr wParam, IntPtr lParam);

    public delegate void TimerProc(IntPtr hWnd, uint uMsg, UIntPtr nIDEvent, uint dwTime);

    private const int WH_CALLWNDPROCRET = 12;

    // ReSharper disable EnumUnderlyingTypeIsInt
    private enum CbtHookAction : int
    // ReSharper restore EnumUnderlyingTypeIsInt
    {
        // ReSharper disable UnusedMember.Local
        HCBT_MOVESIZE = 0,
        HCBT_MINMAX = 1,
        HCBT_QS = 2,
        HCBT_CREATEWND = 3,
        HCBT_DESTROYWND = 4,
        HCBT_ACTIVATE = 5,
        HCBT_CLICKSKIPPED = 6,
        HCBT_KEYSKIPPED = 7,
        HCBT_SYSCOMMAND = 8,
        HCBT_SETFOCUS = 9
        // ReSharper restore UnusedMember.Local
    }

    [DllImport("kernel32.dll")]
    static extern int GetCurrentThreadId();

    [DllImport("user32.dll")]
    private static extern bool GetWindowRect(IntPtr hWnd, ref Rectangle lpRect);

    [DllImport("user32.dll")]
    private static extern int MoveWindow(IntPtr hWnd, int X, int Y, int nWidth, int nHeight, bool bRepaint);

    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int x, int y, int cx, int cy, SetWindowPosFlags uFlags);

    [DllImport("User32.dll")]
    public static extern UIntPtr SetTimer(IntPtr hWnd, UIntPtr nIDEvent, uint uElapse, TimerProc lpTimerFunc);

    [DllImport("User32.dll")]
    public static extern IntPtr SendMessage(IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);

    [DllImport("user32.dll")]
    public static extern IntPtr SetWindowsHookEx(int idHook, HookProc lpfn, IntPtr hInstance, int threadId);

    [DllImport("user32.dll")]
    public static extern int UnhookWindowsHookEx(IntPtr idHook);

    [DllImport("user32.dll")]
    public static extern IntPtr CallNextHookEx(IntPtr idHook, int nCode, IntPtr wParam, IntPtr lParam);

    [DllImport("user32.dll")]
    public static extern int GetWindowTextLength(IntPtr hWnd);

    [DllImport("user32.dll")]
    public static extern int GetWindowText(IntPtr hWnd, StringBuilder text, int maxLength);

    [DllImport("user32.dll")]
    public static extern int EndDialog(IntPtr hDlg, IntPtr nResult);

    [StructLayout(LayoutKind.Sequential)]
    public struct CWPRETSTRUCT
    {
        public IntPtr lResult;
        public IntPtr lParam;
        public IntPtr wParam;
        public uint message;
        public IntPtr hwnd;
    };
    // ReSharper restore InconsistentNaming
}

[Flags]
public enum SetWindowPosFlags : uint
{
    // ReSharper disable InconsistentNaming

    /// <summary>
    ///     If the calling thread and the thread that owns the window are attached to different input queues, the system posts the request to the thread that owns the window. This prevents the calling thread from blocking its execution while other threads process the request.
    /// </summary>
    SWP_ASYNCWINDOWPOS = 0x4000,

    /// <summary>
    ///     Prevents generation of the WM_SYNCPAINT message.
    /// </summary>
    SWP_DEFERERASE = 0x2000,

    /// <summary>
    ///     Draws a frame (defined in the window's class description) around the window.
    /// </summary>
    SWP_DRAWFRAME = 0x0020,

    /// <summary>
    ///     Applies new frame styles set using the SetWindowLong function. Sends a WM_NCCALCSIZE message to the window, even if the window's size is not being changed. If this flag is not specified, WM_NCCALCSIZE is sent only when the window's size is being changed.
    /// </summary>
    SWP_FRAMECHANGED = 0x0020,

    /// <summary>
    ///     Hides the window.
    /// </summary>
    SWP_HIDEWINDOW = 0x0080,

    /// <summary>
    ///     Does not activate the window. If this flag is not set, the window is activated and moved to the top of either the topmost or non-topmost group (depending on the setting of the hWndInsertAfter parameter).
    /// </summary>
    SWP_NOACTIVATE = 0x0010,

    /// <summary>
    ///     Discards the entire contents of the client area. If this flag is not specified, the valid contents of the client area are saved and copied back into the client area after the window is sized or repositioned.
    /// </summary>
    SWP_NOCOPYBITS = 0x0100,

    /// <summary>
    ///     Retains the current position (ignores X and Y parameters).
    /// </summary>
    SWP_NOMOVE = 0x0002,

    /// <summary>
    ///     Does not change the owner window's position in the Z order.
    /// </summary>
    SWP_NOOWNERZORDER = 0x0200,

    /// <summary>
    ///     Does not redraw changes. If this flag is set, no repainting of any kind occurs. This applies to the client area, the nonclient area (including the title bar and scroll bars), and any part of the parent window uncovered as a result of the window being moved. When this flag is set, the application must explicitly invalidate or redraw any parts of the window and parent window that need redrawing.
    /// </summary>
    SWP_NOREDRAW = 0x0008,

    /// <summary>
    ///     Same as the SWP_NOOWNERZORDER flag.
    /// </summary>
    SWP_NOREPOSITION = 0x0200,

    /// <summary>
    ///     Prevents the window from receiving the WM_WINDOWPOSCHANGING message.
    /// </summary>
    SWP_NOSENDCHANGING = 0x0400,

    /// <summary>
    ///     Retains the current size (ignores the cx and cy parameters).
    /// </summary>
    SWP_NOSIZE = 0x0001,

    /// <summary>
    ///     Retains the current Z order (ignores the hWndInsertAfter parameter).
    /// </summary>
    SWP_NOZORDER = 0x0004,

    /// <summary>
    ///     Displays the window.
    /// </summary>
    SWP_SHOWWINDOW = 0x0040,

    // ReSharper restore InconsistentNaming
}

Try this, it's simple enough to justify the time...

This is for Win32 API, written in C. Translate it as you need...

case WM_NOTIFY:{
  HWND X=FindWindow("#32770",NULL);
  if(GetParent(X)==H_frame){int Px,Py,Sx,Sy; RECT R1,R2;
    GetWindowRect(hwnd,&R1); GetWindowRect(X,&R2);
    Sx=R2.right-R2.left,Px=R1.left+(R1.right-R1.left)/2-Sx/2;
    Sy=R2.bottom-R2.top,Py=R1.top+(R1.bottom-R1.top)/2-Sy/2;
    MoveWindow(X,Px,Py,Sx,Sy,1);
  }
} break;

Add that to the WndProc code... You can set position as you like, in this case it just centres over the main program window. It will do this for any messagebox, or file open/save dialog, and likely some other native controls. I'm not sure, but I think you may need to include COMMCTRL or COMMDLG to use this, at least, you will if you want open/save dialogs.

I experimented with looking at the notify codes and hwndFrom of NMHDR, then decided it was just as effective, and far easier, not to. If you really want to be very specific, tell FindWindow to look for a unique caption (title) you give to the window you want it to find.

This fires before the messagebox is drawn onscreen, so if you set a global flag to indicate when action is done by your code, and look for a unique caption, you be sure that actions you take will only occur once (there will likely be multiple notifiers). I haven't explored this in detail, but I managed get CreateWindow to put an edit box on a messagebox dialog. It looked as out of place as a rat's ear grafted onto the spine of a cloned pig, but it works. Doing things this way may be far easier than having to roll your own.

Crow.

EDIT: Small correction to make sure that the right window is handled. Make sure that parent handles agree throughout, and this should work ok. It does for me, even with two instances of the same program...

Surely using your own panel or form would be by far the simplest approach if a little more heavy on the background (designer) code. It gives all the control in terms of centring and manipulation without writing all that custom code.